It's unclear exactly what White House attorneys may be concerned about, but according to CNBC, outside ethics experts point to Bolton's two PACs as potential obstacles.

Garrett Marquis, a spokesman for Bolton, denied that there are any concerns about potential conflicts, telling CNBC that Bolton's transition to the national security adviser role is moving forward with ease.

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"The ambassador's transition to national security adviser is going very smoothly and the ambassador will start on Monday," Marquis said.

Another representative for Bolton told CNBC that the former United Nations ambassador is still meeting with White House lawyers, but did not elaborate on what those meetings entailed.

Bolton's two PACs — the John Bolton PAC and Super PAC — have stopped receiving donations or spending money since he was tapped to replace H.R. McMaster as national security adviser, but have not actually shut down.